1. Field of the Invention
This application relates generally to audio signals and more practically to boosting the bass content in an audio signal.
2. Related Art
The results of Fletcher's and Munson's research, known as the Fletcher-Munson curves are well known in the art and generally teach that as the level of an audio signal is lowered, the responsiveness of the human ear decreases. The results indicate that at lower volume levels, the human ear is less able to hear the lower frequencies (i.e. bass) in the sound. Presently, many audio systems utilize a manual loudness control to boost low and high-end response at low volume levels to compensate for the responsiveness of the human ear.
In FIG. 1, an illustration of a set of frequency domain relative level curves 100 commonly referred to as Robinson-Dadson curves is shown. The Robinson-Dadson curves are the result of more recent studies of how the human ear perceives sound and builds upon the original curves developed by Fletcher and Munson in the early 1930's. These frequency domain relative level curves (equal loudness contours) relate to the frequency response of a human ear to the level of signals being heard. As the signal level decrease, research shows that the responsiveness to the signal by the human ear changes as the bass frequencies decrease.
In FIG. 2, a set of frequency domain relative level curves 200 illustrates the results from the Robinson-Dadson curves for loudness from 10-90 dB relative to the 90 phon curve. The loudness for 10-90 dB is shown with nine curves at 10 dB, 20 dB, 30 dB, 40 dB, 50 dB, 60 dB, 70 dB, 80 dB, and 90 dB. The 90 dB reference for the Robinson-Dadson curves shows that as the loudness decreases below 90 dB that it is desirable to boost the low frequencies.
A known approach to improving the perceived sound quality was proposed in House et al. (U.S. Pat. No. 4,809,338) and implements a bass contour network circuit that is coupled to the program source material. The House et al. patent describes a frequency contour circuit in which the transfer function from source to loudspeaker is altered by a complex attenuation network based on the transfer function of audio reproduction within an automobile. The House et al. patent adds boost to bass frequencies by this approach but the results bare little relationship to Robinson-Dadson curves of FIG. 2. In addition, the House et al. patent measures the signal level at the loudspeaker and thus operates in a feedback mode such that adjustments to the signal frequency content affect the measured signal level forming a servo loop. The House et al. patent uses a passive attenuation system that in reality attenuates mid and high frequencies at low volume levels and fails to describe how to restore that lost signal level and uses an average signal level. Other variants on this scheme utilize notch filters for equalizing the frequency resonance within a bounded area, such as a vehicle's interior. These other variants also use a feedback circuit to detect and adjust bass levels.
In another approach, proposed in the Short et al. patents (U.S. Pat. Nos. 4,739,514 and 5,361,381) circuits are implemented that provide automatic loudness compensation to boost the signal in a bandpass centered at 60 Hz through a circuit that utilizes a 2:1 compressor so that input signals can be compressed, filtered, then re-summed into the forward signal path. Similarly, the Werrbach patent (U.S. Pat. No. 5,359,665) describes a low pass filtered signal applied to a compressor and re-summed into the main signal path. Hence both the Short et al. patent and the Werrbach patent responds only to the signal level in the filtered signal path not the full range signal level.
In the Kimura patent (U.S. Pat. No. 5,172,358), a multiple pass band control scheme is used. In that scheme, the frequency bands are individually processed. Each frequency band is filtered and the level within the frequency band is detected. The detected level within the frequency band is then used to control the boost level applied to that frequency band using a variable boost limited to that frequency band. Contrary to the Fletcher-Munson curves and the Robinson-Dadson curves, the Kimura patent treats loudness as a concept that applies not to the full audible frequency band of the reproduced signal but to sub-bands at both high and low frequencies.
The Iwamura patent (U.S. Pat. No. 5,172,417) describes a three band equalizer that is computed and applied based on reproduced acoustic signal level and applies individual band equalization sections in fixed increments. The Iwamura patent also uses a feedback scheme in which the equalization applied is included in the measured signal that creates a servo-loop in which the compensation chases itself. Further, all these approaches only attempt to simulate the general trend of the Robinson-Dadson curves of FIG. 1 and FIG. 2.
These circuits and other known circuits do not mimic the Robinson-Dadson curves and therefore are not accurately responsive to what a listener can hear. Accordingly, there is a need for a circuit that automatically compensates for the decrease in perceived sound levels at lower volumes by mimicking the Robinson-Dadson curves.